Electronic output transmitters generally operate in either a voltage mode, driving a constant voltage, or a current mode, driving a constant current. As switching frequencies increase, it becomes increasingly important that the characteristics associated with these modes be well controlled in order to minimize adverse interactions in a transmission line environment. At some point, process, voltage, and temperature (PVT) variations make it impossible to statically control the output characteristics of an output transmitter. To improve the output characteristics of an output transmitter beyond this point, it is necessary to calibrate the output transmitter after it is placed into a system environment.
Similarly, transmission line termination means are also subject to PVT variations. As such, the transmission line termination means must be calibrated within a system environment for maximum performance.
Calibration of voltage (V), current (I), and/or resistance (R) for output transmitters or transmission line termination means may be accomplished using known values for V, I, and/or R and some comparison logic. However, there is some difficulty in generating, distributing, and applying the known values to a device being calibrated, while optimizing cost and ease-of-use. For example, applying a reference V and a reference R to calibrate output transmitters operating in current mode requires a resistor and a calibration pin for each output transmitter (or group of output transmitters). This imposes an unacceptable cost and complexity burden on electronic devices such as, for example, memory modules with multiple DRAM devices.
Existing calibration techniques require dedicated external resources in the form of V, I, and/or R sources, along with dedicated pathways (i.e., integrated circuit contacts) to apply them to an electronic device. For systems having many electronic devices, such as, for example, DRAM memory systems, or where calibration precision requires output transmitters and/or transmission line termination means to be calibrated either individually or in small groups, this imposes a large cost and complexity burden upon both the electronic devices and the system.
Existing calibration techniques are typically only performed on systems having few high speed electronic devices. Because of the small number of these electronic devices being calibrated, adding additional components to enable calibration is acceptable, and cost is not a primary design constraint. However, for systems with many data paths and many electronic devices that require calibration, another more cost effective method of calibration is required, reusing existing system elements where possible.
In view of the foregoing, it would be desirable to provide a technique for calibrating electronic devices which overcomes the above-described inadequacies and shortcomings in an efficient and cost effective manner.